Polymerases are enzymes of fundamental importance to living organisms. They are responsible for the synthesis of nucleic acids and their transformation into other nucleic acids necessary for the synthesis of proteins. Polymerases are, therefore, found in all types of cells including the causative DNA virus for hepatitis B virus (HBV).
HBV is a cause of hepatitis, one of the most common infectious human diseases, and the resulting hepatocellular carcinoma (HCC) is one of the most common cancers worldwide. The most effective way in preventing HBV infection and disease to date is through immunization against HBV. The currently licensed HBV vaccines consist of the major surface antigen (HBsAg) in either its natural form (plasma-derived) or recombinant form (purified from yeast cells). The vaccine-induced antibodies have been shown to bind to the most antigenic ‘a’ epitope located within residues 124 to 147 of HBsAg, resulting in effective neutralization of HBV replication.
Although such active vaccination programme has resulted in a decrease of HBV infection in the population, an increasing number of mutations located within the ‘a’ epitope have been emerging. These vaccine-induced HBV mutants are of concern, as they are capable of escaping the currently available immuno-based diagnostic system and able to replicate independently.
The fact that mutations on the ‘a’ epitope of HBsAg give rise to amino acid substitutions in the overlapping HBV DNA polymerase, particularly by their location within the reverse transcriptase domain, may imply that these vaccine-induced mutants have altered reverse transcriptase activity, a key factor for the viral replication.
HBVs are DNA viruses that replicate their genomes by reverse transcription of an RNA intermediate. Packaging of this RNA pregenome into nucleocapsides and initiation of the replication depends primarily on the interaction of the HBV DNA polymerase with the encapsidation signal. There are two copies of the encapsidation signals on the pregenomic RNA. Only the 5′ copy functions as a template for the priming reaction. Following initiation of the minus-strand DNA synthesis, the DNA oligomer (4 nucleotides) is transferred by an unknown mechanism to the 3′ end of pregenomic RNA, where it hybridizes to its complementary sequences. Reverse transcription then continues toward the 5′ end of the RNA template. The interaction between the HBV DNA polymerase and the pregenomic RNA occurs as a covalent link between the N-terminally located tyrosine residue on the DNA polymerase and a specific nucleotide within the encapsidation signal. The HBV reverse transcriptase is one of the four domains within the large DNA polymerase. Other domains include: i) the N-terminal protein, which is responsible for the covalent association of the polymerase to the pregenomic RNA; ii) the spacer region which is tolerant for mutations; and iii) the RNAse H domain involved in the degradation of the mRNA intermediate.
Similarly to other polymerases, detection of the HBV DNA polymerase activity in vitro can be used in the following three situations:                Characterization of a newly isolated virus as a replicative virus and assess the differences with regard to other known viruses. This is particularly relevant for HBV variants with mutations on their surface antigens;        Determination of isolation success of virus from the test material of a subject known to be infected;        Assessment of in vitro efficiency of inhibitors of polymerases that may be antiviral agents.        
Various systems have been established to measure the HBV DNA polymerase activity in intro in the absence of viral replication and other viral proteins. One interesting finding has been the fact that a detectable priming activity of HBV DNA polymerase requires not only the N-terminal protein but also a functional reverse transcriptase domain. One common feature of these systems is therefore the detection of the priming activity of HBV DNA polymerase, indicative of HBV DNA polymerase activity. Two of such systems utilize the duck HBV (DHBV) DNA polymerase, and both have demonstrated reverse transcriptase activity that is template dependent and protein primed. One of the DHBV systems utilizes in vitro translation of DHBV DNA polymerase to obtain lysates that contain a functional DNA polymerase, while the other system packages an active fusion protein of DHBV DNA polymerase in a virus-like particle from the yeast retrotransposon Tyl. Active DNA polymerase has been measured by its priming activity, as indicated by the radio-labeled protein in the presence of priming nucleotide (i.e. [α-32P] dGTP for DHBV DNA polymerase). A similar activity assay has recently been reported for the human HBV DNA polymerase. A 350 base pairs 3′ non-coding region of the polymerase containing the encapsidation signal has been included in all reported constructs, therefore pointing to its importance for in vitro activity assay.
One direct application of the in vitro activity assay for human HBV DNA polymerase may be the screening of novel antiviral agents. Antiviral therapy of chronic HBV infection still remains a problem since several clinical trials have shown that a sustained response to interferon or nucleoside analogs is observed in only 30 to 40% of the patients studied. This response rate is even lower in long-term HBV carriers and in immunocompromised patients. The design of new protocols for chemotherapy to eliminate HBV is needed since the majority of the patients will not clear the viral infection and, therefore, will be at great risk of developing progressive liver disease and hepatocellular carcinoma (HCC). It will also be of particular interest to assess the antiviral effects of such agents on human HBV surface antigen mutants.
Limiting factors exist, however, for antiviral testing using the established in vitro activity assays for human HBV DNA polymerase. One of these has been the cloning step that places the coding region of the HBV DNA polymerase under the control of a viral polymerase promoter (e.g. SP6 or T7) on a plasmid (i.e. pGEM-T) is required in all established systems. In addition, HBV DNA polymerase expressed in some systems requires further purification (i.e. immunoprecipitation) prior to its activity assay. These tedious manipulations are not practical in view of the large number of HBV mutants.